WLANs (Wireless Local Area Networks) utilize RF (Radio Frequency) signals or light signals to connect mobile endpoints to each other or to a centralized gateway and transmit data over a wireless medium between the physical endpoints or between a mobile endpoint and an endpoint on a network that is connected to the WLAN. In 1997 the IEEE published standards for WLANs under the title of 802.11 (also known as “Wi-Fi”). For example, The IEEE 802.11b and 802.11g protocols have gained popularity over the past few years and deployment of networks using these protocols are expected to increase significantly in the near future. Currently, most of these networks are used for data access from laptop computers and personal digital assistants (PDAs) through wireless network interface cards (NICs).
The basic hardware of an IEEE 802.11 network is the BSS (Basic Service Set), which is merely a number of endpoint stations that can communicate with one another or with an access point (AP). In a BSS, IEEE 802.11 enables mobile to communicate, through a wireless network interface, directly with each other or with other STAs through an AP, which is a centralized gateway providing message and power management and access to an external LAN (Local Area Network) and/or the Internet. An Extended Service Set (ESS) can include a combination of BSSs or other network components and nodes.
There exists a plurality of 802.11 standards that each use different frequency bands and have varying data transmission speeds. The original IEEE 802.11 standard supported wireless interfaces operating at speeds of up to 2 megabit per second (Mbps) in the 2.4 GHz radio band. By using different modulation techniques, IEEE 802.11b raised the data transmission rates to 11 Mbps, while 802.11a supports up to 54 Mbps transmission rates at a 5 GHz frequency. The IEEE 802.11g is developing standards for data transmission rates of 54 Mbps at the 2.4 GHz frequency.
WLANs under 802.11 use media access control (MAC) protocols to transmit between wired and wireless devices. Each wireless NIC is assigned a MAC address used to identify the STA. The access to wireless networks is controlled by coordination functions. The distributed coordination function (DCF) provides access similar to Ethernet CSMA/CD access. The DCF determines if the RF link between devices is clear prior to transmitting. Stations use a random backoff after every frame to avoid collisions.
FIG. 1 illustrates a schematic diagram of an exemplary WLAN enterprise network 10. Two wireless APs 12 and 14 are connected to an internal corporate Intranet 18. The Internet 20 may be accessed through intranet 28 or alternatively through APs 12, 14 after registering with a Radius authentication server 22. APs 12, 14 have Multiple APs 12 and 14 provide an enterprise-wide footprint of RF broadcast signals that can be accessed up to a combined geographic range that is represented by coverage ring 24. The extent of coverage and signal power depends upon numerous factors including broadcast signal power, natural signal attenuation, and interferences. An enterprise network typically has multiple APs distributed throughout an office or between multiple buildings so that a mobile station (STA) may access the network from nearly anywhere in the RF broadcast area 24. Since an 802.11 WLAN is traditionally a data network, a wireless endpoint such as laptop computer 16 may access the Internet 20. However, a user may also place and receive phone calls using a WIPP (Wireless Internet Protocol Phone or IP Phone) on WLAN 10 using voice data protocols, such as voice over Internet Protocol (VoIP).
One of the major concerns in 802.11 networks is the limited battery life of 802.11 handheld STAs. The power consumption of a STA depends on the STA's mode of operation. The active mode is defined as the time when the STA is connected to the AP and is in a communication session, i.e. exchanging data with the AP. The standby mode usually refers to the mode where the STA is connected to the AP but there is no ongoing session, i.e. there is no data being exchanged between the STA and the AP.
The 802.11 standard from IEEE (Institute of Electrical and Electronic Engineers) proposes a “doze” mode wherein a STA may “doze” (go to “sleep”) in order to save power, provided the STA informs the AP that the STA is entering the doze mode. This “dozing” algorithm standard significantly decreases the power consumption of the STA both in the active and the standby modes.
Although the 802.11 doze mode is useful for decreasing power consumption in the active and standby modes, there is a mode of a STA that the standard protocol fails to address. This mode is defined herein as the “unconnected” mode. For example, if a STA is out of range of an ESS, there is no AP in the STA's vicinity with which to connect. The default behavior of such an out-of-range STA is to continue scanning for the desired SSID (with the desired security, rates, settings, etc.). Therefore, in the unconnected state, the STA will continue to consume full-power even though there is no active session. This behavior drastically reduces the battery life of the out-of-range STA.